A network that enables transmission of DWDM (Dense Wavelength Division Multiplexing) has been recently constructed in connection with an increase of transmission capacity. Furthermore, ultrahigh-speed products having a transmission speed of 40 Gb/s have been commercially available to respond to a further increase of the information amount, and it has been difficult to compensate a large wavelength dispersion by a optical receiver. Under such a situation, the opportunity for using VDC (Variable Dispersion Compensator) for an individual transponder has increased to satisfy required optical characteristic also in a long-distance transmission.
When VDC is used, various operation sequences based on the presence or absence of an optical input are considered. Furthermore, in the case of using a structure in which a three-dimensional mirror is set to be movable in VDC to vary a wavelength dispersion compensation value, when the three-dimensional mirror is moved at any time, VDC is broken and thus the lifetime of the apparatus itself in which VDC is mounted is shortened because the movable portion is worn away and the lifetime of a position sensor, etc. are relatively short. Therefore, the apparatus must be designed in consideration of this lifetime.
FIG. 1 is a block diagram showing an example of a VDC control circuit in a conventional transponder. In FIG. 1, an optical signal received from a transmission path is supplied to VDC 12 after loss of the optical signal (LOS: Loss Of Signal) thereof is detected in an LOS detector 11.
VDC 12 subjects a received optical signal to wavelength dispersion compensation under the control of the VDC controller 13, and supplies the optical signal concerned to an photoelectric converter (O/E) 14. The optical signal is converted to an electrical signal in the photoelectric converter 14, and a synchronous signal thereof is detected in a synchronous detector (SYNC) 15 and then supplied to a subsequent main signal processor. The synchronous signal is branched at a branch point 16, and supplied to an error detector 17. In the error detector 17, an error number per unit time is detected and then supplied to the VDC controller 13.
The VDC controller 13 varies the wavelength dispersion compensation value of VDC 12 in accordance with increase/decrease of the error number, and an optimum point at which the error is minimum is found and fixed. The VDC controller 13 stops the VDC control when the LOS detector 11 detects LOS (the loss of signal), thereby stopping the movement of the three-dimensional mirror.
Japanese Laid-open Patent Publication No. 03-79141 discloses that a timing signal is extracted from an electrical signal obtained by converting a received optical signal, data are identified from the electrical signal by controlling the phase of the timing signal, and input loss of the optical signal is identified when a phase control signal for controlling the phase exceeds a set range and also when the variation peak value exceeds a predetermined value.